1. Field of the Invention
The present invention relates to an optical transmission system that transmits a signal lightwave comprising a plurality of component signals each having a different wavelength, an optical multiplexer for combining component signals each having a different wavelength to constitute a signal lightwave, and an optical demultiplexer for separating a plurality of component, signals each having a different wavelength from a signal lightwave.
2. Description of the Background Art
The wavelength division multiplexing (WDM) optical transmission system multiplexes a plurality of component signals to transmit them over one optical fiber transmission line. The WDM optical transmission system can transmit and receive a large volume of information over one optical fiber. For trunk-line optical transmission systems for which communication demands are booming, researchers and engineers are studying to increase the transmission capacity further by increasing the degree of multiplexing through the narrowing of the wavelength spacing of the component signals. A WDM transmission system having a high degree of multiplexing is known as a dense WDM (DWDM) system.
Published Japanese patent application 2000-156702 (corresponding published European patent application: EP 0991217A2) has disclosed an invention that relates to a low-cost WDM system having a wide wavelength spacing of the component signals. Such a system is called a coarse WDM (CWDM) system, which is usually used for a relatively low volume of communication traffic. The CWDM optical transmission system has a small number of optical parts, such as signal light sources and light-receiving devices, because it has a small number of component signals. Its wide wavelength spacing of the component signals enables the use of low-cost optical parts, such as optical multiplexers and optical demultiplexers, having low wavelength precision. As a result, a low-cost system can be constructed.
In the CWDM optical transmission system, however, the wide wavelength spacing of the component signals broadens the total wavelength band of a signal lightwave comprising a plurality of component signals. The band width is sometimes broadened to about 100 nm. When the wavelength band has a width of about 100 nm, the optical fiber transmission line has a large difference in transmission loss between an extremely shorter wavelength and an extremely longer wavelength in the band. Consequently, powers of the component signals arriving at the optical receiver differ largely with one another. The difference can be eliminated nearly completely by using an optical amplifier equipped with a gain equalizer. However, the CWDM optical transmission system must be constructed at a low-cost without using the optical amplifier. Therefore, the CWDM optical transmission system has difficulty in transmitting with high quality all the component signals in the total wavelength band of a signal lightwave.